This invention relates to an antenna having a reflector and a primary feed illuminating the reflector, the reflector serving to establish a cross-sectional configuration of a primary beam, wherein the antenna includes a secondary feed comprising an array of feed elements illuminating the reflector to produce a secondary beam with control of sidelobes away from a direction of the primary beam.
An antenna constructed of a reflector illuminated by a feed may be employed in a situation wherein the antenna is required to generate plural beams of electromagnetic radiation. By way of example, in a satellite communication system, a satellite carrying such an antenna encircles the earth in a stationary orbit. The antenna produces the plural beams for simultaneous illumination of plural regions of the earth. Each of the beams has a prescribed cross-sectional configuration for producing a desired footprint at each of the respective illuminated regions of the earth.
A feature in the construction of an antenna comprising a reflector illuminated by a feed is the shaping of the reflector for configuring the rays of radiation from the feed into a beam of desired cross-sectional configuration. This provides optimum efficiency in the transference of electromagnetic power from the feed to the illuminated region.
To develop a second beam angled in direction relative to the primary beam of the primary feed, a secondary feed is positioned for illuminating the reflector, the two feeds being spaced apart so as to introduce the angulation between the two beams. Since the reflector has been configured for optimizing efficiency of the primary feed, the efficiency of transmission of radiant energy from the secondary feed occurs at a lower efficiency. Nevertheless, such an antenna is able to illuminate two separate regions of the earth's surface by the two beams.
However, a problem arises in that the beams may have sidelobes in their respective radiation patterns with the result that a sidelobe of the primary beam may interfere with the propagation of signals from the main lobe of the secondary beam. Similarly, a sidelobe of the secondary beam may be oriented in the direction of the main lobe of the primary beam so as to interfere with the transmission of signals by the primary beam. It is, therefore, desirable to construct the antenna in a manner which avoids interference of the sidelobe of one beam within the main lobe of the other beam. However, a construction of antenna which introduces sufficient isolation of the plural beams has not been available heretofore, and separate antennas have been required for the generation of the separate beams.